Steel sheet with a silvered polymer film laminated to it, and formed to a desirable shape, has gained wide market acceptance for use in lighting fixtures where cost is a secondary consideration, as for example, for light in hospital operating rooms. Relatively less expensive lighting fixtures are made from mild steel painted with a paint containing a white opaque powder having high total reflectance but low distinctness of (reflected) image ("D/I" for brevity). Narrow polished, bright sheets (referred to as "strips") of stainless steel and/or stainless steel clad aluminum (referred to as "bi-metal"), appropriately shaped, are also widely used for decorative trim in automobiles, trucks, boats and a variety of both household and industrial appliances because such decorative trim is eminently durable under aggressive conditions of use. The increasing cost of stainless steel sheet has provided the impetus to replace decorative stainless steel trim with brightened aluminum trim.
The problem is that a brightened, coated and shaped reflective aluminum strip, provided with the protection afforded by any one or more of known coatings, whether inorganic or organic, or both, fails to meet numerous tests which are deemed essential if reflective aluminum trim is to be substituted for the polished stainless steel trim.
This invention relates generally to a shaped, aluminum article having substantially mirror-like characteristics, formed by continuously shaping a "strip" of fluoropolymercoated aluminum alloy, for example, in a roll-forming die, which provides the strip with at least one "tight" radius which is less than 10 mm (0.375 inch). By "substantially mirror-like characteristics" is meant that the surface is characterized by having at least 75% and preferably at least 80% D/I. D/I is expressed as a percentage of specular reflectance R.sub.s. D/I is the sharpness of the reflected image as measured by the ratio of the reflectance at 0.3.degree. from specular to the reflectance at the specular angle, that is, EQU D/I=((R.sub.s -R.sub.0.3)/R.sub.s).times.100%
D/I=0 for a perfect diffuser; D/I=100 for a perfect mirror. Total reflectance of a surface is irrelevant in a consideration of its D/I.
The term "strip" is used herein to specify a relatively narrow and thin sheet of anodized aluminum reflector alloy in the range from about 1 cm to 1 meter wide, preferably from 2 cm to 30 cm wide, and from about 0.5 mm to about 5 mm thick. At least one surface of the shaped article is doubly-protected by a dual-coating consisting essentially of an oxide coating produced by a phosphoric acid (H.sub.3 PO.sub.4) anodizing treatment, the oxide coating, in turn being coated with a cold-workable, environmentally stable, essentially light-permeable coating of a matrix of curable fluoropolymer which is preferably deposited from a solution thereof, on the oxide coating. Hereafter, all references to "aluminum" describe a generally high purity aluminum alloy known, when cleaned and brightened for the purpose at hand with due attention to details of known processes, produces a substantially mirror-like surface
The term "matrix fluoropolymer" is used to highlight the characteristic interchain configuration of the polymer which allows it to be interstitially mechanically bonded to the anodized surface of the reflective aluminum strip, and also to infer that such chain configuration, upon curing of the polymer, produces a receptive substrate which if appropriately treated, will provide a receptive surface in which an adhesive may, in turn, be bonded. Interstitial mechanical bonding is evidenced by chain entanglement of the cured fluoropolymer with a multiplicity of tendrils and pores defined generally by the oxide structure of short columns (schematically illustrated in FIG. 1 and described in greater detail hereafter) which define shallow pores obtained by phosphoric acid anodizing the surface of the reflective aluminum strip. Such chain entanglement is also referred to as a "key-in-lock" structure which allows the anodized surface to grip the surface of the overlaid polymer.
Accordingly, this invention relates to a method of coating a chemically cleaned, chemically brightened but non-etched and anodized strip of mirror-like aluminum alloy with an essentially transparent, durable, weather-resistant, fluoropolymer coating. By "transparent" we refer to a coating which is essentially light-permeable, that is, at least 80% permeable to visible light.
More specifically, this invention relates to the foregoing doubly-protected reflective strip of shaped aluminum which, after being shaped and thereafter being exposed to alternating cycles of ultraviolet (UV) light and 100% humid conditions (commonly referred to as QUV/UVCON) for a prolonged period (i) maintains at least a 80% D/I, and (ii) maintains adhesion of the fluoropolymer coating after the strip is bent in a "Half-T Bend test". In such a test an end portion of the strip is bent double upon the remaining portion, that is, the strip is doubly bent, referred to as a "Zero-T Bend"; the remaining portion is then bent again, first over the end portion, then bent around the small radius formed at the bend of the doubly bent portions of the strip, so that the end portion is sandwiched between the bent portions of the remaining portion (see ASTM D-3794-79). Thus, the "Half-T Bend" is a less stringent test than the "Zero-T Bend" test. The doubly-protected strip of this invention typically meets the more stringent test.
Still more specifically, this invention relates to the foregoing doubly-protected strip, which after being formed to include at least one tight radius, may be laminated to a strip of thermoplastic polymer which is adhesively secured to the exposed surface of the fluoropolymer, provided the surface of the fluoropolymer is treated with a corona (or electric) discharge which "primes" the surface sufficiently to provide interstitial bonding for the adhesive.
Accordingly, this invention also relates to a method of coextruding a strip of electrically primed, polymer-coated reflective aluminum strip and a strip of thermoplastic synthetic resin adhesively bondable thereto, forming laminated decorative trim, for example, automotive trim.
Even organic coatings known to be adherent to smooth, cleaned and brightened surfaces which are conventionally anodized, either do not bond acceptably or do not meet the D/I requirements for the surface of a strip of marketable trim, or both, particularly if such requirements are to be met after exposure outdoors, referred to herein as "aging". Only a curable fluoropolymer, upon being cured, preferably thermally, if acceptably bonded to the strip so that it may be roll-formed, meets the many properties required of decorative reflective aluminum trim.
As will be evident, since the mirror-like surface of substantially pure aluminum must be protected, it is conventionally anodized. However, even a relatively thin (7.6 .mu.m or 0.3 mil) anodized coating formed by phosphoric acid anodizing, after being coated with the most preferred matrix fluoropolymer used in this invention, and acceptably bonded to the coating, is found, upon aging, to "craze" or crack when it is formed or coextruded into an article of arbitrary length and cross section in which at least one radius is less than 10 mm. A sulfuric acid anodized strip of the same aluminum coated with an oxide layer 0.08 mil (2 .mu.m) thick, identically coated with the same fluoropolymer, also shows crazing when bent around a 10 mm mandrel. Because we discovered that only the matrix fluoropolymer coating combines all the necessary qualities to pass the most stringent requirements for such an article, it became necessary to find and provide an anodized coating which was sufficiently thick to afford both, the desired protection and also an adequate key-in-lock structure which would lock in and bond the matrix fluoropolymer. However the combination of anodized coating and fluoropolymer could not be so thick as to vitiate the D/I of the strip, or be unduly susceptible to crazing and cracking after aging.
Surprisingly, when the mirror-like reflective aluminum sheet is protected by an oxide coating produced by phosphoric acid anodizing ("PAA"), under specified conditions, the relatively thin oxide structure produced by short columns which define shallow open pores, affords an excellent grip for the matrix fluoropolymer coating without substantially sacrificing its reflected image clarity and other optical properties, yet is able to withstand a sharp bend without crazing. By "without substantially sacrificing its reflected image clarity" we mean that the D/I measured with a Hunter Lab D-47 DORI-gON (according to ASTM-E430) is decreased by less than 10 percent, preferably less than 5%, when measured within 24 hr after an organic coating at least 0.4 mil thick is dried. By "other optical properties" we refer particularly to specular reflectance "R.sub.s " from which D/I is derived, and, haze, each of which may be measured by the DORI-gON instrument.
Difficult as it is to find an organic coating which does not substantially sacrifice optical properties of the article, it is more difficult to find an organic coating which has excellent weatherability, yet has sufficiently good adhesion on the highly reflective sheet, so that after the sheet is anodized and coated with the organic, the sheet may be shaped into products such as environmentally stable bright-finished product for decorative trim, lighting fixtures and the like, without cracking or crazing either the anodized surface or the organic coating, yet without substantially decreasing the sheet's optical properties.
In a specific application, a coil of the anodized and polymer-coated sheet is cut into strips to make automotive trim. Only one surface is coated with polymer, though both front and rear surfaces may be coated. The coated surface is then roll-formed in progressive rolling dies, cleaned, treated with a corona discharge, and an adhesive applied. In a subsequent step, the adhesive surface is covered with an elastomeric synthetic resinous strip; or, only a portion of a polymer-coated surface may be treated, coated with adhesive and covered with the strip of resin. In a specific embodiment, only those portions of the surface coated with adhesive is covered with an extruded thermoplastic resinous strip.
It is well known that chemical treatments are used to remove soiled and oxidized aluminum surfaces, to brighten them to a specular luster, and to develop various types of protective or decorative coatings. The greatest value of a chemical treatment is as a pretreatment for providing finishes, including organic coatings and laminates, anodizing, electroplating, etc. The adhesion of these finishes, and others, depends in great measure on the type and quality of the chemical pretreatment. A chemical pretreatment may be outstanding as a preparation for paint, but inadequate as a pretreatment for another finish. The result is that, over the years, hundreds of chemical treatments and finishes have been developed to meet diverse needs. (See Aluminum Vol III. Fabricating and Finishing, edited by Kent R. Van Horn, Chapter titled "Chemical Pretreating and Finishing" by George, D. J. et al. pg 587 American Society for Metals, Metals Park, Ohio).
Faced with the problem of making a highly reflective aluminum surface, one skilled in the art typically chooses an aluminum alloy with a known propensity to acquire and retain a high specular luster after being mechanically bright-rolled in coil form. If one starts with such an alloy, it is mechanically bright-rolled to a high luster, cleaned, and then either chemically brightened or electrobrightened, or both. The highly reflective surface thus produced is protected by a thin protective layer of aluminum oxide conventionally deposited by one of several anodizing processes.
Among numerous choices of highly reflective aluminum alloys is the use of one containing from 0.5-3% magnesium, from 0.2-0.5% silver, from 0.001-0.2% iron and from 0.01-0.15% silicon (see U.S. Pat. No. 3,720,508 to Brock et al, class 75/147); and an alloy consisting essentially of 0.25-1.5% Mg (see U.S. Pat. No. 4,601,796 to Powers et al, class 204/33), the balance in each case being aluminum. Because essentially pure aluminum has excellent reflectance, by far the most popular choices for aluminum alloys are those with a low content of alloying elements. Such alloys have inadequate strength for numerous applications which also require a specular reflectance greater than 45%, often greater than 60%. As might be expected, high strength aluminum alloys are not typically chosen for use in high reflectance applications. Yet these alloys of the AA 5XXX and AA 6XXX series, particularly 5657, 5252 and 6306, are the alloys of special interest for use in this invention.
A typical chemical brightening step uses an Alcoa 5 bright dip which comprises dipping the sheet in a hot mixture of 85% phosphoric acid, 70% nitric acid, and optionally, 98% sulfuric acid. Preferably 19 parts (by volume) H.sub.3 PO.sub.4 is mixed with 1 part HNO.sub.3 and from 0 to 0.5 part H.sub.2 SO.sub.4. This ratio varies as the mixture is used repetitively. In addition the brightened surface may be etched in a 30-40% phosphoric acid etch for from 15 sec to about 1 min to ensure formation of a desired semi-specular finish.
The so-obtained reflective surface may be protected by various treatments including anodic oxidation, hydrothermal treatment or conversion coatings employing solutions which may contain chromic acid, chromates, phosphoric acid, phosphates and fluorides. Anodic oxidation, for example, in a sulfuric acid bath, has been the bath of choice since more than a score of years ago (when it was disclosed in U.S. Pat. No. 3,530,048 to Darrow class 204/58). A thinner and more compact coating was provided by the addition of a hydrophilic colloid to the surface during the anodizing step (see U.S. Pat. No. 3,671,333 to Mosier class 204/58). A sulfuric acid anodized coating was favored for a highly reflective coating as recently as five years ago (U.S. Pat. No. 4,601,796 to Powers et al class 204/33).
The approach was to provide as thin a coating as would provide protection without vitiating the specularity of the surface. However, thin oxide coatings of the prior art, no matter how produced on a highly reflective aluminum surface, are far too thick to withstand being sharply bent without "crazing", may provide adequate protection for a short time, but may not provide enough "texture" (familiarly referred to as "grab") to anchor a protective organic coating having excellent durability and optical properties. Further, a thin coating may craze when the strip of aluminum is bent over a 2.5 cm radius mandrel; an anodized coating not quite thin enough will also craze when bent to simulate a forming operation.
In the past, an electrolytic processing step in a phosphoric acid bath, after anodizing in a sulfuric acid bath, was used to provide a surface which was then electrocolored (see U.S. Pat. No. 4,022,671 to Asada class 204/42). But conversion coatings generally have a relatively low D/I because they tend to be colored. Further, conversion coatings provide a less than satisfactory bond, for our purpose, with even the most preferred matrix fluoropolymer.
Another coating on aluminum which was produced with phosphoric acid anodizing followed by AC electrocoloring resulted in a surface with excellent optical properties, as disclosed in French Demande No. 2,360,051 to Showa Aluminum K. K. The process is carried out under constant current conditions of 1 to 1.5 amps/square decimeter. There is no indication as to how bright the sheet is after it is chemically cleaned, nor what the effects of the anodizing and coloring were. There is no indication whether any organic coating would adhere satisfactorily to the surface, least of all a matrix fluoropolymer containing at least 40 mol% of fluoroolefin units, known to produce a cured film of matrix fluoropolymer most difficult to adhere to a smooth metal surface (see U.S. Pat. No. 4,070,525).
Particularly with respect to providing an oxide coating (film) with a phosphoric acid electrolyte, one must achieve a satisfactory balance between anodic coating formation and dissolution of the film in the electrolyte. Sufficient film must be grown to give adequate structural strength to the film and to provide an adequate surface area to give improved adhesion. Equally, dissolution of the film must take place so that the original pore structure is enlarged. However, this attack must not be sufficient to cause breakdown and powdering of the film. With an acid such as phosphoric acid which is capable of strongly attacking the anodic film such a balance is difficult to achieve, particularly when anodizing at high speeds on continuous treatment lines. (See U.S. Pat. No. 4,681,668 to Davies et al, col 2, lines 48-60).
The '668 patent successfully produced a sufficiently thick film from 15 nm to 200 nm thick and required a current density of at least 250 amps/sq.M. As is well known, film growth is controlled essentially by the anodizing current density, and with short contact times such as are available in a bath for continuously treating aluminum strip, one would expect to use a lower current density than 250 A/m.sup.2. But it would seem an exercise in futility to provide such a film in view of the '668 teaching that it would not be sufficiently thick unless a very high current density was used.